In the packaging of certain types of foodstuff products, such as snack foods, it is common practice to use a polymeric packaging film. A highly desirable property for such packaging films is opacity which protects the packaged foodstuff product from deterioration caused by exposure to light. In particular, it has been found that certain wavelengths of light up to about 450 nm cause increased spoilage in the packaged products. Even when a degree of opacity is present in the film, spoilage may occur if the film allows passage of some light. Thus, highly opaque films are desirable for these purposes.
Typically, such opaque polymeric packaging films are multi-layer films comprising an opaque, thermoplastic polymeric core layer having one or more skin layers thereon. The skin layers contribute various desirable characteristics to the packaging film, such as heat sealability, improved appearance, enhanced machine handling capabilities, and the like.
One type of such multi-layer opaque packaging film is disclosed in U.S. Pat. No. 4,632,869 to Park et al. This patent discloses an opaque, biaxially oriented film structure having a polymer matrix with strata of cavitated voids, in which the voids contain spherical void-initiating particles of polybutylene terephthalate (PBT). The structure may also include thermoplastic skin layers, and the film can also include from about 1 to 3% by weight of a pigment such as TiO.sub.2 or colored oxides.
U.S. Pat. No. 4,758,462 to Park et al. discloses an opaque, biaxially oriented film with a cavitated core layer and transparent skin layers. Colored light absorbing pigments such as carbon black or iron oxide are added to the core and/or the skins in an amount of 2 to 12 wt % to decrease light transmission through the film.
In addition, U.S. Pat. No. 4,652,489 to Crass et al. discloses an oriented, sealable, opaque polyolefin multi-layer film with a core layer containing vacuoles or voids, a sealable surface layer, and a non-sealable surface layer which incorporates a slip agent such as a polydiorganosiloxane.
Copending U.S. patent application Ser. No. 08/686,287, filed Jul. 25, 1996, now allowed, which is incorporated herein by reference in its entirety, discloses opaque, oriented polymeric films having void structures which do not require the use of immiscible and incompatible void-initiating particles for void formation and to processes for preparing such films. The process for preparing such films is essentially one in which the polymer (e.g., propylene homopolymer) is stretched rapidly at the start of and simultaneously with crystallization. In one aspect, the process described in the copending application comprises preparing a melt containing a crystallizable polymeric matrix material at a temperature of at least about the melting point of the polymeric matrix material, forming the melt into a sheet containing molten polymeric matrix material, cooling the sheet containing molten material to form a sheet containing amorphous polymeric matrix material and crystallites of the polymeric matrix material, and forming the sheet while containing the amorphous polymeric matrix material into a film by stretching the sheet in at least one direction so as to form voids adjacent to at least some of the crystallites and thereby impart opacity to the film. It is believed that microscopic tears are initiated at the crystalline-amorphous phase interfaces under the applied tensile stress during film production. These tears are enlarged into cavities as the stretching proceeds.
While the above processes for producing opaque polymeric films are generally satisfactory and more than acceptable films can be produced therefrom, there may be several disadvantages or problems associated with the processes. For one, in cases where a void-initiating particle is used to provide opacity, the raw material costs of the void-initiating filler particles are often quite high relative to the raw material costs of the thermoplastic polymeric matrix material. For example, filler particles of PBT, a material which has found commercial acceptance as a void-initiating particle in matrix materials of polyolefins such as polypropylene, are quite expensive on a weight basis as compared to the matrix material, oftentimes up to three times more costly. Further, since the filler particles of materials such as PBT have a density which is greater than that of the matrix material such as polypropylene, the cost penalty per unit volume or area of the resultant film is even greater. For particles of materials other than PBT, the cost differences may be greater or less depending upon the particular material selected.
Additionally, some materials such as PBT, when used as the filler particles, may tend to decompose during extrusion of the film, thereby leaving an objectionable scale or deposit on the surfaces of processing equipment. In the case where PBT decomposes, the deposit may be in the form of an eggshell-like scale on the processing equipment. This scale occasionally breaks up, causing visual defects or breaks in the film in downstream processing. For example, it has been found that a typical line for forming white opaque film using PBT as the void-initiating particles can suffer ten or more hours of downtime per month as a result of these scale formation problems and subsequently necessary die cleanups and the like.
Regardless of the method of cavitation employed, if the film consists of a single cavitated layer or if the cavitated layer(s) is/are external, then the cavities or voids may be close to the surface of the film and even break through the surface. Such a film may have a rough surface, which may be aesthetically objectionable in certain applications. When void-initiating particles are the means of cavitation, it is typical to solve this problem by a film structure comprising at least three layers, where the outermost two layers contain no void-initiating particles and are therefore free of voids. When the method of the above-mentioned copending application Ser. No. 08/686,287, now allowed, is employed to obtain cavitation, a different approach is possible and is the subject of the present invention.